1. Field of the Invention
This invention relates to a process for altering the chemical or physical properties of a resin during production of a reinforced resin part by applying a treated veil to the resin thereby affecting the resin in the proximity of the treated veil during a production process. More particularly, this invention relates to a process for reducing the pulling force necessary to draw material through a mold during fabrication of a reinforced resin article, and to a process for improving the durability of a reinforced resin article when exposed to ultraviolet (UV) light. This invention also relates to the treated veil and to reinforced resin parts made with the treated veil.
2. Description of the Related Art
Fiber reinforced plastic (FRP) structural parts have been successfully used in various applications where the part is subjected to corrosive decay, decomposition, rust, and degradation, such as in chemical plants, paper mills, and plating facilities. These FRP parts are used to replace conventional parts made of, e.g., steel, aluminum, and wood, when there are maintenance problems associated with corrosive environments, or other environments which are not conducive to using conventional parts.
FRP parts or articles can be made by a number of processes, including, but not limited to, the following processes.
Pultrusion is a process for fabricating a reinforced resin product, such as a fiber reinforced plastic (FRP) article. It involves taking various forms of fiber reinforcements (mats, woven products, continuous rovings, etc.) made of materials such as fiberglass, carbon, aramid, etc. which are saturated or wet-out with an uncured thermoset resin. Normally, a polyester resin is used, but it can also be epoxy, phenolic or other resins. These saturated reinforcements are then pulled through a heated, matched metal die or mold machined to the shape of the desired finished part. While in the die or mold, the time and temperature relationship of the die or mold to the resin formulation transforms the resin from a liquid to a solid. This transformation is known as curing, cross-linking or polymerization. During this transformation, exothermic energy is generated in the chemical reaction.
In the pultrusion process, the amount and type of reinforcement to obtain the desired product is first determined. The reinforcement is put in the proper position and held in that position so that a uniform distribution of reinforcement in the resin is achieved. This is accomplished by using the proper amount of tension on the reinforcement along with guiding the reinforcement. If the reinforcement is not uniformly distributed throughout the cross-section of the resin, the finished product could possess areas of structural weakness.
Next, the reinforcements must be saturated or wet-out with a resin in, e.g., the resin tank. Preferably, all of the reinforcements must be wet-out to insure that a quality product is obtained. Viscosity, residence time, and mechanical action are all variables which influence the wet-out process. Without uniform and adequate wet-out, certain areas of the product may be structurally deficient.
Preformers can be used to manipulate the combination of reinforcement and resin in order to reduce die wear and insure uniformity. The combination of reinforcement and resin is then pulled through heated steel dies. Curing occurs during this step of the pultrusion process. Die temperature, pull speed, and the type of catalysts and cure promoter are all variables which control the rate of curing during formation of the product in the dies. The pull speed remains constant during the pultrusion process. Different shapes will require different speed settings.
The finished pultrusion product is then cut to the desired size.. The resulting product possesses outstanding strength to weight ratio. See FIG. 1 for a schematic design of a typical pultrusion process.
Contact molding or open molding are other FRP processes. Resins and reinforcements are manually (hand lay-up) or mechanically (spray-up) deposited on an open mold surface. The mold surface is preferably previously coated with a gel coat and is provided with a surfacing veil such as a mat or fabric. Once the required amounts of reinforcements and resin have been deposited on the mold, the laminate is worked with rollers, brushes or squeegees, usually manually, to remove any trapped air and to thoroughly saturate or wet-out the reinforcements with resin. Once this is completed, the laminate is allowed to cure, normally at ambient temperature.
Resin transfer molding (RTM) and structural reaction injection molding (S-RIM) are two similar closed mold FRP processes in which the required reinforcement package, including a surfacing veil such as a mat or fabric, is placed on one-half of the mold cavity, usually the bottom half. Once properly positioned, the top half of the mold is closed on the bottom half and secured in place. Next, the resin is injected slowly under minimal (e.g. 50 psi) pressure in RTM or rapidly under high pressure (e.g. 2000 psi) in S-RIM. The mechanical pumping and resulting pressure cause the air to be flushed out of the mold cavity and the resin to saturate or wet-out the reinforcement. The resin impregnated reinforced article is then allowed to cure.
Compression molding is also an FRP mold process. In this process, the reinforcement package including surfacing veil (mat(s) or fabric(s)) and the resin are placed on one-half, usually the bottom half, of the mold cavity. Once properly positioned, the top half of the mold is mechanically closed on the bottom half using a press which compresses the reinforcement package and resin under pressure (from 50 to 1500 psi) to flush out the air and thoroughly saturate or wet-out the reinforcement package with resin. It is then cured normally with the assistance of heat.
Filament winding is an FRP process in which reinforcements, normally continuous rovings, are saturated with resin, normally by pulling them through a pan or bath containing the resin. The reinforcements are then wound on a rotating mandrel in a specific pattern. The mandrel may or may not have been previously covered with a resin impregnated surfacing veil. One or more outer layers of surfacing veil may be wrapped over the resin impregnated reinforcement when required. Once the required amount of resin, reinforcements and surfacing veils are properly placed on the mandrel, the laminate is allowed to cure with or without the assistance of heat.
The continuous panel process is an FRP process for making continuous flat and/or shaped, e.g., corrugated, panels. It involves depositing a resin on a carrier film which then passes under a reinforcement deposition area. Various types of reinforcement are then applied to the film or resin. The reinforcement and resin then go through a compaction section where a series of belts, screens, or rollers force air out and thoroughly saturate or wet-out the reinforcement with resin. A surfacing veil such as a mat or fabric may be placed on either the top or bottom surface of the resulting saturated material and the veil is allowed to be saturated with resin. A carrier film is then applied to the top surface of the resulting article which is passed through a curing station where the resin cures normally with the assistance of heat. Once cured, the carrier film is removed and the article is cut to the desired length.
A publication of Fiberglas Canada, Inc. entitled "An Introduction to Fiberglas-Reinforced Plastics/Composites" provides a detailed overview of FRP production. The teachings of this publication are hereby incorporated by reference into this application.
Mold-release agents may used to prevent molded FRP parts from sticking to mold surfaces. Mold-release agents may be classified as internal or external, depending upon the method of their use. Internal additives are added directly to the resin, while external additives are applied directly to the mold surfaces. Presently, the use of external additives is much greater than the use of internal additives. Mold-release agents are further discussed in the 1994 edition of the Modern Plastics Encyclopedia, herein incorporated by reference. External mold additives are generally combined with solvents and propellants that are used to apply the additives to the mold surface. Use of organic materials as solvents and propellants can lead to environmental problems. For example, chlorinated fluorocarbons (CFCs) have traditionally been used as propellants in external mold-release agents. However, emissions concerns have led to the prohibition of the use of CFCs as propellants. Likewise, more stringent controls on the emissions of volatile organic compounds (VOCs) have led to a desire to reduce their use in mold-release applications.
UV stabilizers and UV absorbers are used in FRP articles to prevent degradation due to the action of UV light on UV sensitive materials in the FRP articles. UV absorbers, such as benzophenone-based materials, may act by absorbing the UV light, and thus preventing the light from reacting with, for example, the resin in the FRP article. UV stabilizers such as hindered amine light stabilizers (HALS), do not directly absorb the UV light, but rather may stabilize the resin by absorbing free-radicals created by the UV light. UV stabilizers and UV absorbers are further discussed in the 1994 edition of the Modern Plastics Encyclopedia, herein incorporated by reference.
Cure promoters are often added in FRP processes. Resin reactivity in FRP processes is controlled by a wide variety of properties. The base resin, as supplied by the resin manufacturer, will vary in reactivity based on formulation, viscosity, temperature in storage, age, etc. The curing of a resin is based on cross-linking the individual molecules to form long chain molecules. Cross-linking can be achieved by the use of a catalyst and/or heat. The rate of cross-linking is determined by how fast the catalyst disassociates into free radicals of active oxygen, which initiates the dross-linking.
The catalyst's rate of disassociation into free radicals can be controlled by heat and/or promoters. It is possible to reduce curing time and/or heating requirements for the manufacture of resin materials by adding a promoter to the resin and curing agent. More heat and/or promoter results in more free radicals, faster cross-linking, faster cure and higher exotherm temperatures. This produces faster cure and faster line speeds. Too high an exotherm temperature, however, causes the finished part to be structurally weaker.